In a vehicle seat having a back frame made of a non-ferrous material such as a composite resin material, when the back frame is coupled to a reclining mechanism, there is a structure in which the back frame is pressed against a recliner by a plate member and installed. In the technique described in JP-A-2017-39483, a through hole, into which a projection portion provided on a surface of the recliner facing a side frame is loosely fitted, is provided on a side frame lower end portion of the back frame made of the composite resin material. The plate member is provided with a burring portion to which the projection portion is fitted from an inner side thereof and an outer side thereof is fitted into the through hole. The plate member is fixed to the recliner in a state where the outer side of the burring portion is fitted to the through hole of the side frame and the projection portion of the recliner is fitted to the inner side the burring portion.
In the technique described in JP-A-2017-39483, a torsional moment applied from the side frame to the recliner is received only at a fitting portion between the outer side of the burring portion and the through hole of the side frame. Therefore, there is a problem that when attempting to increase the torsional moment in which the side frame can endure, it is necessary to increase thickness of the side frame, increase reinforcing fibers contained in a composite resin material constituting the side frame, etc. Accordingly, it is difficult to achieve weight reduction.